ACPAtmospheric Chemistry and PhysicsACPAtmos. Chem. Phys.1680-7324Copernicus PublicationsGöttingen, Germany10.5194/acp-13-11935-2013Undisturbed and disturbed above canopy ponderosa pine emissions: PTR-TOF-MS measurements and MEGAN 2.1 model resultsKaserL.12KarlT.28GuentherA.2GrausM.34SchnitzhoferR.1TurnipseedA.2FischerL.5HarleyP.2MadronichM.2GochisD.6KeutschF. N.7HanselA.11Institute for Ion Physics and Applied Physics, University of Innsbruck, Innsbruck, Austria2Atmospheric Chemistry Division, National Center for Atmospheric Research, Boulder, CO, USA3Chemical Sciences Division, NOAA Earth System Research Laboratory, Boulder, CO, USA4Cooperative Institute for Research in Environmental Sciences (CIRES), University of Colorado, Boulder, CO, USA5Ionimed Analytik GmbH, Innsbruck, Austria6Research Application Laboratory, National Center for Atmospheric Research, Boulder, CO, USA7Department of Chemistry, University of Wisconsin-Madison, Madison, WI, USA8now at: Institute of Meteorology and Geophysics, University of Innsbruck, Innsbruck, Austria0912201313231193511947This work is licensed under a Creative Commons Attribution 3.0 Unported License. To view a copy of this license, visit http://creativecommons.org/licenses/by/3.0/This article is available from http://www.atmos-chem-phys.net/13/11935/2013/acp-13-11935-2013.htmlThe full text article is available as a PDF file from http://www.atmos-chem-phys.net/13/11935/2013/acp-13-11935-2013.pdf

We present the first eddy covariance flux measurements of volatile organic
compounds (VOCs) using a proton-transfer-reaction time-of-flight
mass spectrometer (PTR-TOF-MS) above a ponderosa pine forest in Colorado,
USA. The high mass resolution of the PTR-TOF-MS enabled the identification
of chemical sum formulas. During a 30 day measurement period in August and
September 2010, 649 different ion mass peaks were detected in the ambient
air mass spectrum (including primary ions and mass calibration compounds).
Eddy covariance with the vertical wind speed was calculated for all ion mass
peaks. On a typical day, 17 ion mass peaks, including protonated parent
compounds, their fragments and isotopes as well as VOC-H<sup>+</sup>-water
clusters, showed a significant flux with daytime average emissions above a
reliable flux threshold of 0.1 mg compound m<sup>−2</sup> h<sup>−1</sup>. These ion mass
peaks could be assigned to seven compound classes. The main flux
contributions during daytime (10:00–18:00 LT) are attributed to the sum of
2-methyl-3-buten-2-ol (MBO) and isoprene (50%), methanol (12%), the
sum of acetic acid and glycolaldehyde (10%) and the sum of monoterpenes
(10%). The total MBO + isoprene flux was composed of 10% isoprene and
90% MBO.
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There was good agreement between the light- and temperature dependency of the
sum of MBO and isoprene observed for this work and those of earlier studies.
The above canopy flux measurements of the sum of MBO and isoprene and the
sum of monoterpenes were compared to emissions calculated using the Model of
Emissions of Gases and Aerosols from Nature (MEGAN 2.1). The best agreement
between MEGAN 2.1 and measurements was reached using emission factors
determined from site-specific leaf cuvette measurements. While the modeled
and measured MBO + isoprene fluxes agree well, the emissions of the sum of
monoterpenes is underestimated by MEGAN 2.1. This is expected as some
factors impacting monoterpene emissions, such as physical damage of needles
and branches due to storms, are not included in MEGAN 2.1.
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After a severe hailstorm event, 22 ion mass peaks (attributed to six
compound classes plus some unknown compounds) showed an elevated flux for
the two following days. The sum of monoterpene emissions was 4–23 times
higher compared to emissions prior to the hailstorm while MBO emissions
remained unchanged. The monoterpene emission (in mg compound m<sup>&minus;2</sup>)
during this measurement period is underestimated by 40% if the effect of
this disturbance source is not considered.